A geophone is a sensor that is often employed for seismic survey operations, typically as part of an extensive array of such sensors. Geophones normally take the form of a mass-spring system, in which a spring-mounted mass is movable with respect to a housing. The mass may be magnetic in nature or the geophone may be equipped with a magnet that provides a magnetic field, which facilitates monitoring of the mass's relative motion by an inductive coil attached to the housing. As seismic waves encounter the geophone, they displace the mass relative to the housing. So long as the displacement frequency is within the geophone's response range, this displacement causes the spring-mounted mass to move relative to the housing. (An example of a typical geophone response range would be 3-40 Hz. Other illustrative ranges include 20-200 Hz, and 3-13 kHz.) The motion of the magnetic mass relative to the inductive coil generates a voltage at the coil's terminals proportional to the relative velocity of the mass. The resonance frequency or frequencies of the mass-spring system may be chosen to improve the transducer's response to a selected frequency, but such frequencies are often a fixed parameter of the geophone design. Frequencies outside the transducer's range are substantially attenuated.
One approach that may be employed to address this issue in a controlled survey environment is to employ a design that provides a very broad response curve. However, such an approach may be unsuitable for use in a seismic-while-drilling (SWD) application, where the sensor must endure high-intensity shocks and vibrations from the drilling environment. A geophone having a broad response curve may be expected to have frequent high-energy collisions between the spring mounted mass and the sensor housing, with a correspondingly fast degradation in sensor performance, or conversely, may be expected to be so heavily damped that it is unsuitable for use as a seismic sensor. The nature of SWD application requires the geophone to be used in varying depth in the borehole. This results in a trade-off in selecting sensors of suitable resonance frequency. In deeper well, lower frequency seismic waves experiences lower attenuation while in shallower well, high frequency waves provides higher resolution.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.